Linux 3.0.3

[wandboard.git] / mm / backing-dev.c

#include <linux/wait.h>
#include <linux/backing-dev.h>
#include <linux/kthread.h>
#include <linux/freezer.h>
#include <linux/fs.h>
#include <linux/pagemap.h>
#include <linux/mm.h>
#include <linux/sched.h>
#include <linux/module.h>
#include <linux/writeback.h>
#include <linux/device.h>
#include <trace/events/writeback.h>

static atomic_long_t bdi_seq = ATOMIC_LONG_INIT(0);

struct backing_dev_info default_backing_dev_info = {
        .name           = "default",
        .ra_pages       = VM_MAX_READAHEAD * 1024 / PAGE_CACHE_SIZE,
        .state          = 0,
        .capabilities   = BDI_CAP_MAP_COPY,
};
EXPORT_SYMBOL_GPL(default_backing_dev_info);

struct backing_dev_info noop_backing_dev_info = {
        .name           = "noop",
        .capabilities   = BDI_CAP_NO_ACCT_AND_WRITEBACK,
};
EXPORT_SYMBOL_GPL(noop_backing_dev_info);

static struct class *bdi_class;

/*
 * bdi_lock protects updates to bdi_list and bdi_pending_list, as well as
 * reader side protection for bdi_pending_list. bdi_list has RCU reader side
 * locking.
 */
DEFINE_SPINLOCK(bdi_lock);
LIST_HEAD(bdi_list);
LIST_HEAD(bdi_pending_list);

static struct task_struct *sync_supers_tsk;
static struct timer_list sync_supers_timer;

static int bdi_sync_supers(void *);
static void sync_supers_timer_fn(unsigned long);

#ifdef CONFIG_DEBUG_FS
#include <linux/debugfs.h>
#include <linux/seq_file.h>

static struct dentry *bdi_debug_root;

static void bdi_debug_init(void)
{
        bdi_debug_root = debugfs_create_dir("bdi", NULL);
}

static int bdi_debug_stats_show(struct seq_file *m, void *v)
{
        struct backing_dev_info *bdi = m->private;
        struct bdi_writeback *wb = &bdi->wb;
        unsigned long background_thresh;
        unsigned long dirty_thresh;
        unsigned long bdi_thresh;
        unsigned long nr_dirty, nr_io, nr_more_io;
        struct inode *inode;

        nr_dirty = nr_io = nr_more_io = 0;
        spin_lock(&inode_wb_list_lock);
        list_for_each_entry(inode, &wb->b_dirty, i_wb_list)
                nr_dirty++;
        list_for_each_entry(inode, &wb->b_io, i_wb_list)
                nr_io++;
        list_for_each_entry(inode, &wb->b_more_io, i_wb_list)
                nr_more_io++;
        spin_unlock(&inode_wb_list_lock);

        global_dirty_limits(&background_thresh, &dirty_thresh);
        bdi_thresh = bdi_dirty_limit(bdi, dirty_thresh);

#define K(x) ((x) << (PAGE_SHIFT - 10))
        seq_printf(m,
                   "BdiWriteback:     %8lu kB\n"
                   "BdiReclaimable:   %8lu kB\n"
                   "BdiDirtyThresh:   %8lu kB\n"
                   "DirtyThresh:      %8lu kB\n"
                   "BackgroundThresh: %8lu kB\n"
                   "b_dirty:          %8lu\n"
                   "b_io:             %8lu\n"
                   "b_more_io:        %8lu\n"
                   "bdi_list:         %8u\n"
                   "state:            %8lx\n",
                   (unsigned long) K(bdi_stat(bdi, BDI_WRITEBACK)),
                   (unsigned long) K(bdi_stat(bdi, BDI_RECLAIMABLE)),
                   K(bdi_thresh), K(dirty_thresh),
                   K(background_thresh), nr_dirty, nr_io, nr_more_io,
                   !list_empty(&bdi->bdi_list), bdi->state);
#undef K

        return 0;
}

static int bdi_debug_stats_open(struct inode *inode, struct file *file)
{
        return single_open(file, bdi_debug_stats_show, inode->i_private);
}

static const struct file_operations bdi_debug_stats_fops = {
        .open           = bdi_debug_stats_open,
        .read           = seq_read,
        .llseek         = seq_lseek,
        .release        = single_release,
};

static void bdi_debug_register(struct backing_dev_info *bdi, const char *name)
{
        bdi->debug_dir = debugfs_create_dir(name, bdi_debug_root);
        bdi->debug_stats = debugfs_create_file("stats", 0444, bdi->debug_dir,
                                               bdi, &bdi_debug_stats_fops);
}

static void bdi_debug_unregister(struct backing_dev_info *bdi)
{
        debugfs_remove(bdi->debug_stats);
        debugfs_remove(bdi->debug_dir);
}
#else
static inline void bdi_debug_init(void)
{
}
static inline void bdi_debug_register(struct backing_dev_info *bdi,
                                      const char *name)
{
}
static inline void bdi_debug_unregister(struct backing_dev_info *bdi)
{
}
#endif

static ssize_t read_ahead_kb_store(struct device *dev,
                                  struct device_attribute *attr,
                                  const char *buf, size_t count)
{
        struct backing_dev_info *bdi = dev_get_drvdata(dev);
        char *end;
        unsigned long read_ahead_kb;
        ssize_t ret = -EINVAL;

        read_ahead_kb = simple_strtoul(buf, &end, 10);
        if (*buf && (end[0] == '\0' || (end[0] == '\n' && end[1] == '\0'))) {
                bdi->ra_pages = read_ahead_kb >> (PAGE_SHIFT - 10);
                ret = count;
        }
        return ret;
}

#define K(pages) ((pages) << (PAGE_SHIFT - 10))

#define BDI_SHOW(name, expr)                                            \
static ssize_t name##_show(struct device *dev,                          \
                           struct device_attribute *attr, char *page)   \
{                                                                       \
        struct backing_dev_info *bdi = dev_get_drvdata(dev);            \
                                                                        \
        return snprintf(page, PAGE_SIZE-1, "%lld\n", (long long)expr);  \
}

BDI_SHOW(read_ahead_kb, K(bdi->ra_pages))

static ssize_t min_ratio_store(struct device *dev,
                struct device_attribute *attr, const char *buf, size_t count)
{
        struct backing_dev_info *bdi = dev_get_drvdata(dev);
        char *end;
        unsigned int ratio;
        ssize_t ret = -EINVAL;

        ratio = simple_strtoul(buf, &end, 10);
        if (*buf && (end[0] == '\0' || (end[0] == '\n' && end[1] == '\0'))) {
                ret = bdi_set_min_ratio(bdi, ratio);
                if (!ret)
                        ret = count;
        }
        return ret;
}
BDI_SHOW(min_ratio, bdi->min_ratio)

static ssize_t max_ratio_store(struct device *dev,
                struct device_attribute *attr, const char *buf, size_t count)
{
        struct backing_dev_info *bdi = dev_get_drvdata(dev);
        char *end;
        unsigned int ratio;
        ssize_t ret = -EINVAL;

        ratio = simple_strtoul(buf, &end, 10);
        if (*buf && (end[0] == '\0' || (end[0] == '\n' && end[1] == '\0'))) {
                ret = bdi_set_max_ratio(bdi, ratio);
                if (!ret)
                        ret = count;
        }
        return ret;
}
BDI_SHOW(max_ratio, bdi->max_ratio)

#define __ATTR_RW(attr) __ATTR(attr, 0644, attr##_show, attr##_store)

static struct device_attribute bdi_dev_attrs[] = {
        __ATTR_RW(read_ahead_kb),
        __ATTR_RW(min_ratio),
        __ATTR_RW(max_ratio),
        __ATTR_NULL,
};

static __init int bdi_class_init(void)
{
        bdi_class = class_create(THIS_MODULE, "bdi");
        if (IS_ERR(bdi_class))
                return PTR_ERR(bdi_class);

        bdi_class->dev_attrs = bdi_dev_attrs;
        bdi_debug_init();
        return 0;
}
postcore_initcall(bdi_class_init);

static int __init default_bdi_init(void)
{
        int err;

        sync_supers_tsk = kthread_run(bdi_sync_supers, NULL, "sync_supers");
        BUG_ON(IS_ERR(sync_supers_tsk));

        setup_timer(&sync_supers_timer, sync_supers_timer_fn, 0);
        bdi_arm_supers_timer();

        err = bdi_init(&default_backing_dev_info);
        if (!err)
                bdi_register(&default_backing_dev_info, NULL, "default");
        err = bdi_init(&noop_backing_dev_info);

        return err;
}
subsys_initcall(default_bdi_init);

int bdi_has_dirty_io(struct backing_dev_info *bdi)
{
        return wb_has_dirty_io(&bdi->wb);
}

static void bdi_flush_io(struct backing_dev_info *bdi)
{
        struct writeback_control wbc = {
                .sync_mode              = WB_SYNC_NONE,
                .older_than_this        = NULL,
                .range_cyclic           = 1,
                .nr_to_write            = 1024,
        };

        writeback_inodes_wb(&bdi->wb, &wbc);
}

/*
 * kupdated() used to do this. We cannot do it from the bdi_forker_thread()
 * or we risk deadlocking on ->s_umount. The longer term solution would be
 * to implement sync_supers_bdi() or similar and simply do it from the
 * bdi writeback thread individually.
 */
static int bdi_sync_supers(void *unused)
{
        set_user_nice(current, 0);

        while (!kthread_should_stop()) {
                set_current_state(TASK_INTERRUPTIBLE);
                schedule();

                /*
                 * Do this periodically, like kupdated() did before.
                 */
                sync_supers();
        }

        return 0;
}

void bdi_arm_supers_timer(void)
{
        unsigned long next;

        if (!dirty_writeback_interval)
                return;

        next = msecs_to_jiffies(dirty_writeback_interval * 10) + jiffies;
        mod_timer(&sync_supers_timer, round_jiffies_up(next));
}

static void sync_supers_timer_fn(unsigned long unused)
{
        wake_up_process(sync_supers_tsk);
        bdi_arm_supers_timer();
}

static void wakeup_timer_fn(unsigned long data)
{
        struct backing_dev_info *bdi = (struct backing_dev_info *)data;

        spin_lock_bh(&bdi->wb_lock);
        if (bdi->wb.task) {
                trace_writeback_wake_thread(bdi);
                wake_up_process(bdi->wb.task);
        } else {
                /*
                 * When bdi tasks are inactive for long time, they are killed.
                 * In this case we have to wake-up the forker thread which
                 * should create and run the bdi thread.
                 */
                trace_writeback_wake_forker_thread(bdi);
                wake_up_process(default_backing_dev_info.wb.task);
        }
        spin_unlock_bh(&bdi->wb_lock);
}

/*
 * This function is used when the first inode for this bdi is marked dirty. It
 * wakes-up the corresponding bdi thread which should then take care of the
 * periodic background write-out of dirty inodes. Since the write-out would
 * starts only 'dirty_writeback_interval' centisecs from now anyway, we just
 * set up a timer which wakes the bdi thread up later.
 *
 * Note, we wouldn't bother setting up the timer, but this function is on the
 * fast-path (used by '__mark_inode_dirty()'), so we save few context switches
 * by delaying the wake-up.
 */
void bdi_wakeup_thread_delayed(struct backing_dev_info *bdi)
{
        unsigned long timeout;

        timeout = msecs_to_jiffies(dirty_writeback_interval * 10);
        mod_timer(&bdi->wb.wakeup_timer, jiffies + timeout);
}

/*
 * Calculate the longest interval (jiffies) bdi threads are allowed to be
 * inactive.
 */
static unsigned long bdi_longest_inactive(void)
{
        unsigned long interval;

        interval = msecs_to_jiffies(dirty_writeback_interval * 10);
        return max(5UL * 60 * HZ, interval);
}

static int bdi_forker_thread(void *ptr)
{
        struct bdi_writeback *me = ptr;

        current->flags |= PF_SWAPWRITE;
        set_freezable();

        /*
         * Our parent may run at a different priority, just set us to normal
         */
        set_user_nice(current, 0);

        for (;;) {
                struct task_struct *task = NULL;
                struct backing_dev_info *bdi;
                enum {
                        NO_ACTION,   /* Nothing to do */
                        FORK_THREAD, /* Fork bdi thread */
                        KILL_THREAD, /* Kill inactive bdi thread */
                } action = NO_ACTION;

                /*
                 * Temporary measure, we want to make sure we don't see
                 * dirty data on the default backing_dev_info
                 */
                if (wb_has_dirty_io(me) || !list_empty(&me->bdi->work_list)) {
                        del_timer(&me->wakeup_timer);
                        wb_do_writeback(me, 0);
                }

                spin_lock_bh(&bdi_lock);
                set_current_state(TASK_INTERRUPTIBLE);

                list_for_each_entry(bdi, &bdi_list, bdi_list) {
                        bool have_dirty_io;

                        if (!bdi_cap_writeback_dirty(bdi) ||
                             bdi_cap_flush_forker(bdi))
                                continue;

                        WARN(!test_bit(BDI_registered, &bdi->state),
                             "bdi %p/%s is not registered!\n", bdi, bdi->name);

                        have_dirty_io = !list_empty(&bdi->work_list) ||
                                        wb_has_dirty_io(&bdi->wb);

                        /*
                         * If the bdi has work to do, but the thread does not
                         * exist - create it.
                         */
                        if (!bdi->wb.task && have_dirty_io) {
                                /*
                                 * Set the pending bit - if someone will try to
                                 * unregister this bdi - it'll wait on this bit.
                                 */
                                set_bit(BDI_pending, &bdi->state);
                                action = FORK_THREAD;
                                break;
                        }

                        spin_lock(&bdi->wb_lock);

                        /*
                         * If there is no work to do and the bdi thread was
                         * inactive long enough - kill it. The wb_lock is taken
                         * to make sure no-one adds more work to this bdi and
                         * wakes the bdi thread up.
                         */
                        if (bdi->wb.task && !have_dirty_io &&
                            time_after(jiffies, bdi->wb.last_active +
                                                bdi_longest_inactive())) {
                                task = bdi->wb.task;
                                bdi->wb.task = NULL;
                                spin_unlock(&bdi->wb_lock);
                                set_bit(BDI_pending, &bdi->state);
                                action = KILL_THREAD;
                                break;
                        }
                        spin_unlock(&bdi->wb_lock);
                }
                spin_unlock_bh(&bdi_lock);

                /* Keep working if default bdi still has things to do */
                if (!list_empty(&me->bdi->work_list))
                        __set_current_state(TASK_RUNNING);

                switch (action) {
                case FORK_THREAD:
                        __set_current_state(TASK_RUNNING);
                        task = kthread_create(bdi_writeback_thread, &bdi->wb,
                                              "flush-%s", dev_name(bdi->dev));
                        if (IS_ERR(task)) {
                                /*
                                 * If thread creation fails, force writeout of
                                 * the bdi from the thread.
                                 */
                                bdi_flush_io(bdi);
                        } else {
                                /*
                                 * The spinlock makes sure we do not lose
                                 * wake-ups when racing with 'bdi_queue_work()'.
                                 * And as soon as the bdi thread is visible, we
                                 * can start it.
                                 */
                                spin_lock_bh(&bdi->wb_lock);
                                bdi->wb.task = task;
                                spin_unlock_bh(&bdi->wb_lock);
                                wake_up_process(task);
                        }
                        break;

                case KILL_THREAD:
                        __set_current_state(TASK_RUNNING);
                        kthread_stop(task);
                        break;

                case NO_ACTION:
                        if (!wb_has_dirty_io(me) || !dirty_writeback_interval)
                                /*
                                 * There are no dirty data. The only thing we
                                 * should now care about is checking for
                                 * inactive bdi threads and killing them. Thus,
                                 * let's sleep for longer time, save energy and
                                 * be friendly for battery-driven devices.
                                 */
                                schedule_timeout(bdi_longest_inactive());
                        else
                                schedule_timeout(msecs_to_jiffies(dirty_writeback_interval * 10));
                        try_to_freeze();
                        /* Back to the main loop */
                        continue;
                }

                /*
                 * Clear pending bit and wakeup anybody waiting to tear us down.
                 */
                clear_bit(BDI_pending, &bdi->state);
                smp_mb__after_clear_bit();
                wake_up_bit(&bdi->state, BDI_pending);
        }

        return 0;
}

/*
 * Remove bdi from bdi_list, and ensure that it is no longer visible
 */
static void bdi_remove_from_list(struct backing_dev_info *bdi)
{
        spin_lock_bh(&bdi_lock);
        list_del_rcu(&bdi->bdi_list);
        spin_unlock_bh(&bdi_lock);

        synchronize_rcu();
}

int bdi_register(struct backing_dev_info *bdi, struct device *parent,
                const char *fmt, ...)
{
        va_list args;
        struct device *dev;

        if (bdi->dev)   /* The driver needs to use separate queues per device */
                return 0;

        va_start(args, fmt);
        dev = device_create_vargs(bdi_class, parent, MKDEV(0, 0), bdi, fmt, args);
        va_end(args);
        if (IS_ERR(dev))
                return PTR_ERR(dev);

        bdi->dev = dev;

        /*
         * Just start the forker thread for our default backing_dev_info,
         * and add other bdi's to the list. They will get a thread created
         * on-demand when they need it.
         */
        if (bdi_cap_flush_forker(bdi)) {
                struct bdi_writeback *wb = &bdi->wb;

                wb->task = kthread_run(bdi_forker_thread, wb, "bdi-%s",
                                                dev_name(dev));
                if (IS_ERR(wb->task))
                        return PTR_ERR(wb->task);
        }

        bdi_debug_register(bdi, dev_name(dev));
        set_bit(BDI_registered, &bdi->state);

        spin_lock_bh(&bdi_lock);
        list_add_tail_rcu(&bdi->bdi_list, &bdi_list);
        spin_unlock_bh(&bdi_lock);

        trace_writeback_bdi_register(bdi);
        return 0;
}
EXPORT_SYMBOL(bdi_register);

int bdi_register_dev(struct backing_dev_info *bdi, dev_t dev)
{
        return bdi_register(bdi, NULL, "%u:%u", MAJOR(dev), MINOR(dev));
}
EXPORT_SYMBOL(bdi_register_dev);

/*
 * Remove bdi from the global list and shutdown any threads we have running
 */
static void bdi_wb_shutdown(struct backing_dev_info *bdi)
{
        if (!bdi_cap_writeback_dirty(bdi))
                return;

        /*
         * Make sure nobody finds us on the bdi_list anymore
         */
        bdi_remove_from_list(bdi);

        /*
         * If setup is pending, wait for that to complete first
         */
        wait_on_bit(&bdi->state, BDI_pending, bdi_sched_wait,
                        TASK_UNINTERRUPTIBLE);

        /*
         * Finally, kill the kernel thread. We don't need to be RCU
         * safe anymore, since the bdi is gone from visibility. Force
         * unfreeze of the thread before calling kthread_stop(), otherwise
         * it would never exet if it is currently stuck in the refrigerator.
         */
        if (bdi->wb.task) {
                thaw_process(bdi->wb.task);
                kthread_stop(bdi->wb.task);
        }
}

/*
 * This bdi is going away now, make sure that no super_blocks point to it
 */
static void bdi_prune_sb(struct backing_dev_info *bdi)
{
        struct super_block *sb;

        spin_lock(&sb_lock);
        list_for_each_entry(sb, &super_blocks, s_list) {
                if (sb->s_bdi == bdi)
                        sb->s_bdi = &default_backing_dev_info;
        }
        spin_unlock(&sb_lock);
}

void bdi_unregister(struct backing_dev_info *bdi)
{
        if (bdi->dev) {
                bdi_set_min_ratio(bdi, 0);
                trace_writeback_bdi_unregister(bdi);
                bdi_prune_sb(bdi);
                del_timer_sync(&bdi->wb.wakeup_timer);

                if (!bdi_cap_flush_forker(bdi))
                        bdi_wb_shutdown(bdi);
                bdi_debug_unregister(bdi);
                device_unregister(bdi->dev);
                bdi->dev = NULL;
        }
}
EXPORT_SYMBOL(bdi_unregister);

static void bdi_wb_init(struct bdi_writeback *wb, struct backing_dev_info *bdi)
{
        memset(wb, 0, sizeof(*wb));

        wb->bdi = bdi;
        wb->last_old_flush = jiffies;
        INIT_LIST_HEAD(&wb->b_dirty);
        INIT_LIST_HEAD(&wb->b_io);
        INIT_LIST_HEAD(&wb->b_more_io);
        setup_timer(&wb->wakeup_timer, wakeup_timer_fn, (unsigned long)bdi);
}

int bdi_init(struct backing_dev_info *bdi)
{
        int i, err;

        bdi->dev = NULL;

        bdi->min_ratio = 0;
        bdi->max_ratio = 100;
        bdi->max_prop_frac = PROP_FRAC_BASE;
        spin_lock_init(&bdi->wb_lock);
        INIT_LIST_HEAD(&bdi->bdi_list);
        INIT_LIST_HEAD(&bdi->work_list);

        bdi_wb_init(&bdi->wb, bdi);

        for (i = 0; i < NR_BDI_STAT_ITEMS; i++) {
                err = percpu_counter_init(&bdi->bdi_stat[i], 0);
                if (err)
                        goto err;
        }

        bdi->dirty_exceeded = 0;
        err = prop_local_init_percpu(&bdi->completions);

        if (err) {
err:
                while (i--)
                        percpu_counter_destroy(&bdi->bdi_stat[i]);
        }

        return err;
}
EXPORT_SYMBOL(bdi_init);

void bdi_destroy(struct backing_dev_info *bdi)
{
        int i;

        /*
         * Splice our entries to the default_backing_dev_info, if this
         * bdi disappears
         */
        if (bdi_has_dirty_io(bdi)) {
                struct bdi_writeback *dst = &default_backing_dev_info.wb;

                spin_lock(&inode_wb_list_lock);
                list_splice(&bdi->wb.b_dirty, &dst->b_dirty);
                list_splice(&bdi->wb.b_io, &dst->b_io);
                list_splice(&bdi->wb.b_more_io, &dst->b_more_io);
                spin_unlock(&inode_wb_list_lock);
        }

        bdi_unregister(bdi);

        for (i = 0; i < NR_BDI_STAT_ITEMS; i++)
                percpu_counter_destroy(&bdi->bdi_stat[i]);

        prop_local_destroy_percpu(&bdi->completions);
}
EXPORT_SYMBOL(bdi_destroy);

/*
 * For use from filesystems to quickly init and register a bdi associated
 * with dirty writeback
 */
int bdi_setup_and_register(struct backing_dev_info *bdi, char *name,
                           unsigned int cap)
{
        char tmp[32];
        int err;

        bdi->name = name;
        bdi->capabilities = cap;
        err = bdi_init(bdi);
        if (err)
                return err;

        sprintf(tmp, "%.28s%s", name, "-%d");
        err = bdi_register(bdi, NULL, tmp, atomic_long_inc_return(&bdi_seq));
        if (err) {
                bdi_destroy(bdi);
                return err;
        }

        return 0;
}
EXPORT_SYMBOL(bdi_setup_and_register);

static wait_queue_head_t congestion_wqh[2] = {
                __WAIT_QUEUE_HEAD_INITIALIZER(congestion_wqh[0]),
                __WAIT_QUEUE_HEAD_INITIALIZER(congestion_wqh[1])
        };
static atomic_t nr_bdi_congested[2];

void clear_bdi_congested(struct backing_dev_info *bdi, int sync)
{
        enum bdi_state bit;
        wait_queue_head_t *wqh = &congestion_wqh[sync];

        bit = sync ? BDI_sync_congested : BDI_async_congested;
        if (test_and_clear_bit(bit, &bdi->state))
                atomic_dec(&nr_bdi_congested[sync]);
        smp_mb__after_clear_bit();
        if (waitqueue_active(wqh))
                wake_up(wqh);
}
EXPORT_SYMBOL(clear_bdi_congested);

void set_bdi_congested(struct backing_dev_info *bdi, int sync)
{
        enum bdi_state bit;

        bit = sync ? BDI_sync_congested : BDI_async_congested;
        if (!test_and_set_bit(bit, &bdi->state))
                atomic_inc(&nr_bdi_congested[sync]);
}
EXPORT_SYMBOL(set_bdi_congested);

/**
 * congestion_wait - wait for a backing_dev to become uncongested
 * @sync: SYNC or ASYNC IO
 * @timeout: timeout in jiffies
 *
 * Waits for up to @timeout jiffies for a backing_dev (any backing_dev) to exit
 * write congestion.  If no backing_devs are congested then just wait for the
 * next write to be completed.
 */
long congestion_wait(int sync, long timeout)
{
        long ret;
        unsigned long start = jiffies;
        DEFINE_WAIT(wait);
        wait_queue_head_t *wqh = &congestion_wqh[sync];

        prepare_to_wait(wqh, &wait, TASK_UNINTERRUPTIBLE);
        ret = io_schedule_timeout(timeout);
        finish_wait(wqh, &wait);

        trace_writeback_congestion_wait(jiffies_to_usecs(timeout),
                                        jiffies_to_usecs(jiffies - start));

        return ret;
}
EXPORT_SYMBOL(congestion_wait);

/**
 * wait_iff_congested - Conditionally wait for a backing_dev to become uncongested or a zone to complete writes
 * @zone: A zone to check if it is heavily congested
 * @sync: SYNC or ASYNC IO
 * @timeout: timeout in jiffies
 *
 * In the event of a congested backing_dev (any backing_dev) and the given
 * @zone has experienced recent congestion, this waits for up to @timeout
 * jiffies for either a BDI to exit congestion of the given @sync queue
 * or a write to complete.
 *
 * In the absence of zone congestion, cond_resched() is called to yield
 * the processor if necessary but otherwise does not sleep.
 *
 * The return value is 0 if the sleep is for the full timeout. Otherwise,
 * it is the number of jiffies that were still remaining when the function
 * returned. return_value == timeout implies the function did not sleep.
 */
long wait_iff_congested(struct zone *zone, int sync, long timeout)
{
        long ret;
        unsigned long start = jiffies;
        DEFINE_WAIT(wait);
        wait_queue_head_t *wqh = &congestion_wqh[sync];

        /*
         * If there is no congestion, or heavy congestion is not being
         * encountered in the current zone, yield if necessary instead
         * of sleeping on the congestion queue
         */
        if (atomic_read(&nr_bdi_congested[sync]) == 0 ||
                        !zone_is_reclaim_congested(zone)) {
                cond_resched();

                /* In case we scheduled, work out time remaining */
                ret = timeout - (jiffies - start);
                if (ret < 0)
                        ret = 0;

                goto out;
        }

        /* Sleep until uncongested or a write happens */
        prepare_to_wait(wqh, &wait, TASK_UNINTERRUPTIBLE);
        ret = io_schedule_timeout(timeout);
        finish_wait(wqh, &wait);

out:
        trace_writeback_wait_iff_congested(jiffies_to_usecs(timeout),
                                        jiffies_to_usecs(jiffies - start));

        return ret;
}
EXPORT_SYMBOL(wait_iff_congested);